There are three types of the relationship between 3GPP and WLAN. The first one is no-coupling, that means the mobility is non-seamless. The second one is loose-coupling (3GPP TR 37.824), and IFOM (IP Flow Mobility) is used for the 3GPP/WLAN interworking. However, due to the integration point is at P-GW, it may cost a long time and cause a large overhead at mobility. The third one is tight-coupling, i.e. RAN-level Aggregation (3GPP RP-142281), and the eNB is responsible for the bearer switch/split to offload traffic to WLAN.
For the mobility issue, the authentication procedure of WLAN is lengthy, since AP selection is decided by a user equipment. The association costs tens of milliseconds and the authentication costs one to a few seconds. This is a big challenge to support seamless services, such as VoIP, in a multi-RAT of LTE/WLAN.